Since I retired I've had to do some of these online courses/webinars to get enough PDH's to keep my PE License current. In my experience a webinar that is truly free doesn't offer and PDH's. Some courses offer as an option a quiz that also involves a fee and that type of course will generally get you PDH's once you successfully complete the test.
Yes, you need to show proof of attendance usually by providing a certificate from the webinar organizers. Make sure to keep all your certificates for your records.